The present invention relates to an ultrasonic beam former used in ultrasonic tomography units of electronic scanning type, for example. For receiving the ultrasonic wave coming from the desired position on the section of the object in conformity to the wave front, electric signals produced from respective elements of the transducer array are controlled in phase and added together in the ultrasonic beam former which is composed of wave receiving and phasing circuits. In the phase circuits of the conventional ultrasonic tomography unit of electronic scanning type, lump constant (LC) delay lines were used.
In a wave receiving and phasing circuit described in Japanese Patent Unexamined Publication No. 41142/83 (JP-A-58-141142), for example, sampling delay means such as a sample-and-hold circuit is used instead of the LC delay line. In this circuit using the sampling delay means, respective received signals are sampled at a frequency higher than twice the highest frequency within the signal band. The signal values thus sampled are held during time periods corresponding to respective delay time values and added together to receive and phase ultrasonic wave signals.
If a higher frequency is used as the ultrasonic frequency to yield images with high resolution, however, the sampling must also be effected at a higher frequency. Because of a limit in the operation speed of the sampling element, therefore, it was difficult to apply the above described receiving and phasing circuit to a high ultrasonic frequency.
Further, the received signal supplied from the transducer element has an extremely high dynamic range. Since the dynamic range of the sampling element is generally narrow, however, it was difficult to obtain a tomographic image of wide range in the depth direction with a high signal-to-noise ratio.